In general, as a vulcanization accelerator for rubber, a thiuram-based, sulfenamide-based, mercaptobenzothiazole-based and other accelerators have been used. A sulfenamide-based accelerator is a delayed action accelerator. It is said that, during heating, the N—S bonds is dissociated by heat and mercaptobenzothiazole and amine are regenerated. It is known that the regenerated mercaptobenzothiazole acts as a vulcanization accelerator and the amine is coordinated to the zinc oxide and, therefore, plays an important role in accelerating the vulcanization reaction by activation of the vulcanization system and reaction with the vulcanization intermediates (see Chapman, A. V., Porter, M.: “Sulphur Vulcanization Chemistry” in the Natural Rubber Science and Technology, Roberts, A. D. Ed., Oxford Science Publications, London (1988)).
On the other hand, dibenzothiazole disulfide, which is a disulfide-based vulcanization agent, mercaptobenzothiazole is regenerated due to the dissociation of S—S bonds by heat but has no vulcanization activation capability due to amine, and, therefore, is said to be slow in accelerating vulcanization and inferior in vulcanization accelerating capability compared with sulfenamides. It may be considered to jointly use amines for the purpose of improving the vulcanization accelerating capability of dibenzothiazole disulfide, but in such a case there is the problem that, since free amines are high in reactivity, they react with the vulcanization agents such as sulfur even at a low temperature and have a detrimental effect on the scorch time.
The bonding performance between metal belts and rubber in pneumatic tires is, of course, important from the viewpoint of the fact that tires are composites. If this bonding performance is low, troubles such as tire separation are caused. As a countermeasure to this, the techniques of compounding of cobalt (Co) salts and the change of the vulcanization accelerator have been tried to make the bonding reaction dominant (see the Journal of SRIJ, vol. 65, p. 86 (1992)), but there are the problems of deterioration of the low heat buildup property.